Polyamic acid varnish composition and a flexible printed board

ABSTRACT

A polyamic acid varnish composition, which is composed of a polyamic acid obtained by the addition polymerization of an diamine and an aromatic acid dianhydride, contains as an additive a specific imidazolyl-diaminoazine.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a polyamic acid varnishcomposition which is intended to form a polyimide insulating layer of aflexible printed board, and a flexible printed board which employs same.

[0003] 2. Description of the Related Art

[0004] Flexible printed boards in which polyimide insulating layersformed directly onto a metal foil, e.g., copper foil, without anadhesive being interposed therebetween are manufactured by applying apolyamic acid varnish composition, which is obtained through theaddition polymerization of an aromatic diamine such as aparaphenylenediamine and an aromatic acid dianhydride such aspyromellitic dianhydride in a solvent such as N-methyl-2-pyrrolidone,onto the copper foil, drying the whole to obtain a polyamic acid layer(polyimide precursor layer), and then heating and imidizing it at300-400° C. to form a polyimide insulating layer.

[0005] However, the presence of carboxyl groups in the polyamic acidgives rise to such problems as the corrosion and discoloration of thecopper foil surface when the polyamic acid varnish is applied to thecopper foil as described in the foregoing, as well as the formation ofcopper ions which bring about electrical migration in the flexibleprinted wiring board.

[0006] Accordingly, an imidazole-based rust inhibitor (e.g., AdekastubCDA-1; manufactured by Asahi Denka) is generally added to polyamic acidvarnishes which are used upon manufacture of flexible printed boards.

[0007] However, conventionally used rust inhibitors dissolve poorly inthe polyamic acid varnish, and furthermore, excessive blooming occurs onthe polyimide surface under the high temperature heating duringimidation, resulting in a deterioration in the adhesive strength of thepolyimide with respect to the copper foil. Moreover, the rust-inhibitoris scattered during imidation, forming a vapor residue substance thatdeposits on the imidation process apparatus and on the flexible printedboard products and contaminates them.

SUMMARY OF THE INVENTION

[0008] In an attempt to resolve the aforedescribed problems in the priorart, it is an object of the present invention to provide a polyamic acidvarnish composition which is free of problems such as blooming or vaporresidue substance formation occurring during the polyamic acidimidation, and electrical migration, and furthermore, from which it ispossible to form a polyimide insulating layer which demonstratessatisfactory adhesive strength with respect to copper foil.

[0009] The present inventors perfected the present invention based onthe discovery that a polyamic acid varnish composition, which isobtained by adding a specific imidazolyl-diaminoazine into a mixturewhich contains a polyamic acid obtained by the addition polymerizationof an aromatic diamine and an aromatic acid dianhydride in a solvent,can achieve the aforesaid objective.

[0010] In other words, the present invention provides a polyamic acidvarnish composition, comprising a polyamic acid obtained by the additionpolymerization of an aromatic diamine and an aromatic acid dianhydride,and a solvent, wherein said polyamic acid varnish composition containsas an additive an imidazolyl-diaminoazine as represented by the Formula(1)

[0011] (In the formula, A is an imidazolyl group as represented byFormulae (1a), (1b) or (1c)

[0012] R¹ is an alkylene group, and m is 0 or 1. R is an alkyl group andn is 0, 1, or 2. R³ and R⁴ are alkylene groups, and p and q arerespectively 0 or 1. B is an azine, diazine or triazine residue.)

[0013] Moreover, the present invention provides a flexible printed boardhaving a polyimide insulating layer furnished on a metal foil, whereinthe polyimide insulating layer is formed by forming a film of theaforementioned polyamic acid varnish composition on the metal foil,followed by imidizing.

DETAILED DESCRIPTION OF THE INVENTION

[0014] The polyamic acid varnish composition pertaining to the presentinvention has in common with conventional polyamic acid varnishcompositions the aspect that it is constituted from a polyamic acidobtained by the addition polymerization of an aromatic diamine and anaromatic acid dianhydride, and a solvent. However, the polyamic acidvarnish composition pertaining to the present invention is characterisedby the aspect that it contains as an additive an imidazolyl-diaminoazinerepresented by Formula (1), in whose molecules is contained an imidazoleresidue which has a rust-inhibiting effect. Accordingly, the polyamicacid varnish composition pertaining to the present invention exhibits noproblems such as blooming or vapor residue substance formation duringthe imidation of the polyamic acid, nor electrical migration, andtherefrom a polyimide insulating layer can be formed which exhibitssatisfactory adhesive strength with respect to copper foil. The exactreasons for this are not clear, but it is presumably due to theimidazolyl-diaminoazine, which is represented in Formula (1) andcompounded as an additive, being taken into the polyimide created fromthe reaction with the polyamic acid during imidation.

[0015] In the imidazolyl-diaminoazine as represented by Formula (1),when m is 0, there are no alkylene groups R¹ and the imidazole ring andthe azine, diazine or triazine residue are directly bonded. Methylene,ethylene, propylene or the like can be cited as the alkylene groups R¹when m is 1.

[0016] When n is 0, there are no alkyl groups R² and a hydrogen atom isbonded to the imidazole ring. Methyl, ethyl or the like can be cited asthe alkyl groups R² when n is 1. When n is 2, two R² groups are bondedto the imidazole ring, and methyl, ethyl or the like may beindependently cited as examples of each type of alkyl group R². R² mayoccasionally be directly bonded to a nitrogen atom of the imidazolering.

[0017] When p is 0, there are no alkylene groups R³ and the amino groupis directly bonded to the azine, diazine or triazine residue. Methylene,ethylene or the like can be cited as examples of the alkylene groups R³when p is 1.

[0018] When q is 0, there are no alkylene groups R⁴ and the amino groupis directly bonded to the azine, diazine or triazine residue. Methylene,ethylene or the like can be cited as examples of the alkylene groups R⁴when q is 1.

[0019] B represents an azine, diazine or triazine residue. Among these,a diamine which contains a triazine residue is preferable because ofease of synthesis and commercial availability.

[0020] The following compounds can be cited as specific examples ofpreferable imidazolyl-diaminoazines as represented by Formula (1), whenp and q are both 0.

[0021] 2,4-diamino-6-[2-(2-methyl-1-imidazolyl)ethyl]-s-triazine;

[0022] 2,4-diamino-6-[2-(2-ethyl-1-imidazolyl)ethyl]-s-triazine;

[0023] 2,4-diamino-6-[1-(2-undecyl-1-imidazolyl)ethyl]-s-triazine;

[0024] 2,4-diamino-6-[2-(2- imidazolyl)ethyl]-s-triazine;

[0025] 2,4-diamino-6-[2-(1-imidazolyl)ethyl]-s-triazine;

[0026] 2,4-diamino-6-(2-ethyl-4-imidazolyl)-s-triazine;

[0027] 2,4-diamino-6-[2-(4-methyl-1-imidazolyl)ethyl]-s-triazine;

[0028] 2,4-diamino-6-(2-ethyl-5-methyl-4-imidazolyl)-s-triazine;

[0029] 2,4-diamino-6-(4-ethyl-2-methyl-1-imidazolyl)-s-triazine;

[0030] 2,4-diamino-6-[3-(2-methyl-1-imidazolyl)propyl]-s-triazine;

[0031] 2,4-diamino-6-[4-(2-imidazolyl)butyl]-s-triazine;

[0032] 2,4-diamino-6-[2-(2-methyl-1-imidazolyl)propyl]-s-triazine;

[0033]2,4-diamino-6-[1-methyl-2-(2-methyl-1-imidazolyl)ethyl]-s-triazine;

[0034] 2,4-diamino-6-[2-(2,5-dimethyl-1-imidazolyl)ethyl]-s-triazine;

[0035] 2,4-diamino-6-[2-(2,4-dimethyl-1-imidazolyl)ethyl]-s-triazine; or

[0036]2,4-diamino-6-[2-(2-ethyl-4-methyl-1-imidazolyl)ethyl]-s-triazine.

[0037] Of these, the following are considered as preferable:

[0038]2,4-diamino-6-[2-(2-ethyl-4-methyl-1-imidazolyl)ethyl]-s-triazine;

[0039] 2,4-diamino-6-[2-(2-methyl-1-imidazolyl)ethyl]-s-triazine; and

[0040] 2,4-diamino-6-[1-(2-undecyl-1-imidazolyl)ethyl]-s-triazine.

[0041] The imidazolyl-diaminoazine represented by Formula (1) compoundedin the polyamic acid varnish composition will result in the polyimideinsulating layer exhibiting insufficient adhesive strength when presentin excess or insufficient amounts, and moreover, the mechanical strengthand heat resistance will decrease in the event of an excess amount.Therefore, it is preferable for the content to be 0.1 to 10 wt parts,and more preferably 0.1 to 5 wt parts, per 100 wt parts polyamic acid,which constitutes the solid component.

[0042] According to the present invention, any known conventionalaromatic diamine can be preferably cited for use as the aromatic diaminecomponent of the polymeric acid. For instance,4,4′-diaminodiphenylether, paraphenylenediamine,4,4′-diaminobenzanilide, 4,4′-bis(p-aminophenoxy)diphenylsulfone and2,2-bis[4-(4-aminophenoxy)phenyl]propane can be cited as preferredexamples.

[0043] Of the aforesaid aromatic diamines, paraphenylenediamine ispreferably used in order to reduce the thermal expansion properties ofthe polyimide. 4,4′-diaminodiphenyl ether is preferably used in order toincrease the thermal expansion properties of the polyimide.

[0044] Any known aromatic acid dianhydride can be preferably cited foruse as the aromatic acid dianhydride component of the polyamic acid. Forinstance, pyromellitic dianhydride (PMDA),3,4,3′,4′-biphenyltetracarboxylic dianhydride (BPDA),3,4,3′,4′-benzophenonetetracarboxylic dianhydride (BTDA), or3,4,3′,4′-diphenylsulfonetetracarboxylic dianhydride (DSDA) can be citedas preferred examples.

[0045] The ratio of aromatic diamine, which constitutes the polyamicacid, used with respect to the aromatic acid dianhydride is not affectedby whether there is an excess of aromatic diamines or aromatic aciddianhydrides, though it is preferable for them to be used in anequimolar ratio.

[0046] Any solvent which is used in conventional known polyamic acidvarnish compositions may be used as the solvent in the polyamic acidvarnish composition pertaining to the present invention;N-methyl-2-pyrrolidone being a preferred example.

[0047] There is no particular restriction on the amount of solvent to beused. It can be suitably determined in accordance with the viscosityetc. of the polyamic acid varnish composition.

[0048] Any known conventional additive may be compounded with thepolyamic acid varnish composition pertaining to the present invention,according to need.

[0049] The polyamic acid varnish composition pertaining to the presentinvention is obtained by the addition polymerization of an aromaticdiamine and an aromatic acid dianhydride in a solvent. The additionpolymerization conditions can be suitably determined in accordance withpolyamic acid addition polymerization conditions as used whenconventionally performed. Specifically, first, an aromatic diamine isheated and dissolved in a solvent (e.g., N-methyl-2-pyrrolidone), andaddition polymerization is conducted over several hours while an aciddianhydride is gradually added in an atmosphere of inert gas such asnitrogen at 0-90° C. and preferably 5-50° C. Polyamic acid dissolved inthe solvent is thus obtained. The polyamic acid varnish compositionpertaining to the present invention can be obtained by adding, mixingand dissolving an imidazolyl-diaminoazine, as represented by Formula(1), into the solution.

[0050] A substrate with a structure whereby a polyimide insulating layerhas been furnished on a metal foil can be cited for use as the flexibleprinted board obtained when using the polyamic acid varnish compositionpertaining to the present invention.

[0051] The polyimide insulating layer is formed by forming the polyamicacid varnish composition pertaining to the present invention as a filmand subjecting it to imidation. Setting the coefficient of thermalexpansion of the polyimide used in the present invention at or slightlyabove the coefficient of thermal expansion of the metal foil used ispreferable in terms of preventing the flexible printed board (and awiring board fabricated therefrom) from undergoing excessive curling (ora convexity from being formed on the polyimide side due to curling). Thecoefficient of thermal expansion of the polyimide can be adjustedthrough combining diamines and acid dianhydrides, as disclosed inJapanese Patent Application Laid-Open No. 60-157286.

[0052] There is no particular restriction on the thickness of thepolyimide insulating layer, with a thickness of 10-50 μm being common.

[0053] Various types of metal foil may be used for this flexible printedboard. Preferred examples thereof include aluminium foil, copper foiland gold foil. These metal foils may also be appropriately matted,plated or treated with aluminium alcoholates, aluminium chelates, silanecoupling agents or the like.

[0054] There is no particular restriction on the thickness of the metalfoil, with a thickness of 5-35 μm being common.

[0055] The flexible printed board as described above can be manufacturedas described hereinbelow.

[0056] First, the polyamic acid varnish composition pertaining to thepresent invention is applied to a metal foil using a comma coater, thendried thereupon, resulting in a polyamic acid layer which serves as apolyimide precursor. It is preferable for the amount of residualvolatile content (content of remaining solvent undried and of watergenerated upon imidation) to be kept to 70% or less in the dryingprocess so as to prevent foaming during the subsequent imidation step.

[0057] The resulting polyamic acid layer is heated in an inertatmosphere (e.g., a nitrogen atmosphere) at 300-400° C. and imidized toform an insulating layer composed of a polyimide. A flexible printedboard is obtained from this process.

[0058] In flexible printed boards so obtained, neither corrosion nordiscoloration will occur on the surface of the copper or other metalfoil (polyimide formation surface). Moreover, no electrical migrationwill be caused by copper ions when the flexible printed board is used asa wiring board. No blooming will occur on the polyimide surface from therust inhibitor, and no formation of vapor residue substance will occurduring imidation. Accordingly the adhesive strength between thepolyimide layer and the copper foil will be satisfactory.

EXAMPLES

[0059] The present invention shall be described in detail below.

[0060] Examples 1 through 5 and Comparative Examples 1 through 2

[0061] 83.3 g (0.77 mol) paraphenylenediamine (PDA; manufactured byDaishin Kasei) and 46.0 g (0.23 mol) 4,4′-diaminodiphenylether (DPE;manufactured by Wakayama Seika) were dissolved in approximately 3 kg ofthe solvent N-methyl-2-pyrrolidone (NMP; manufactured by MitsubishiChemical) in a 5 L jacketed reaction vessel under a nitrogen gas ambientatmosphere and the solution was maintained at 50° C. 297.1 g (1.01 mol)3,4,3′,4′-biphenyltetracarboxylic dianhydride (BPDA; manufactured byMitsubishi Chemical) was then gradually added therein and a reactionallowed to occur for three hours.

[0062] 21.3 g2,4-diamino-6-[2-(2-ethyl-4-methyl-1-imidazolyl)ethyl-s-triazine as theimidazolyl-diaminoazine (5 wt parts to 100 wt parts polyamic acid) wasadded to the resulting polyamic acid solution and dissolved therein,resulting in a polyamic acid varnish.

[0063] The polyamic acid varnishes pertaining to Examples 2 through 5and Comparative Examples 1 through 2 were obtained by repeating the sameprocedure of Example 1, in accordance with the compounding ratios shownin Table 1.

[0064] The compounds mentioned in Table 1 are as follows: (aromatic aciddianhydrides) BPDA: 3,4,3′,4′-biphenyltetracarboxylic dianhydride PMDA:pyromellitic dianhydride (aromatic diamines) PDA: paraphenylenediamineDPE: 4,4′-diaminodiphenylether DABA: 4,4′-diaminobenzanilide

[0065] TABLE 1 Weight parts Acid Diamine Molar ratio of FormulaDianhydride (a) (b) (a)/(b) (1) compd. Example 1 BPDA PDA DPE 75/25 5 2BPDA PDA DPE 75/25 2 3 BPDA PDA DPE 75/25 10  4 BPDA PDA DPE 75/25 3 5PMDA PDA DABA 30/70 5 Comparative Example 1 BPDA PDA DPE 75/25 — 2 PMDAPDA DABA 30/70 —

[0066] Next, the polyamic acid varnishes pertaining to Examples 1through 5 and Comparative Examples 1 through 2 were applied onto copperfoils (SQ-VLP 12 μm electrolytic foils; Mitsui Kinzoku) which had beensoft-etched with 2% HCl and dried, resulting in a polyamic acid layers10 μm in thickness. The resulting laminates were placed in an atmosphereof 40° C. and 90% RH, and visually assessed as to whether the surfacesof the copper foils had suffered any discoloration. The results areshown in table 2.

[0067] Next, the same polyamic varnishes were applied ontosurface-roughened copper foils (manufactured by Furukawa Circuit Foil(KK)) and dried step by step so as to prevent foaming, whereuponflexible printed boards which had a 25 μm-thick polyimide insulatinglayer were obtained by imidizing the laminates in a nitrogen atmosphereat 350° C. (10 min).

[0068] The copper foil of the resulting flexible printed boards waspatterned to form parallel circuits with a conductor interval of 0.1 mm,yielding wiring boards. The wiring boards were allowed to stand for 7days in an 85° C./90% RH atmosphere while a DC voltage of 50V wasapplied between adjacent conductor patterns. Thereafter, the resistancebetween the adjacent conductor patterns was measured, and electricalmigration was also measured. A resistance of 10⁷Ω or higher wasconsidered acceptable.

[0069] The peel strength (kg/cm) of the polyimide insulating layers ofthe flexible printed boards was also measured as adhesive strength at23° C. according to JIS C 6471 (peeling in a 90° direction at a width of1.59 mm). The results are shown in Table 2. TABLE 2 Rust preventingElectrical Adhesive strength effects (discoloration) migration (23° C.)(kg/cm) Examples 1 none pass 1.43 2 none pass 1.46 3 none pass 1.50 4none pass 1.35 5 none pass 1.36 Comparative Example 1 rapiddiscoloration fail (shorting) 0.74 2 rapid discoloration fail (shorting)0.63

[0070] As can be understood from the results in Table 2, the flexibleprinted boards (Examples 1 through 5), which contained polyimideinsulating layers formed from polyamic acid varnishes which contained aspecific imidazolyl-diaminoazine as a separate additive, exhibited anexcellent rust preventing effect even though a rust-inhibitor was notused. Moreover, the data show that no electrical migration occurred, andthat the adhesive strength between the polyimide insulating layer andthe copper foil was satisfactory.

[0071] Conversely, the flexible printed boards (Comparative Examples 1through 2), which contained polyimide insulating layers formed frompolyamic acid varnishes which did not contain a specificimidazolyl-diaminoazine as a separate additive, exhibited no rustinhibiting effect, exhibited electrical migration and had inadequateadhesive strength between the polyimide insulating layer and the copperfoil.

[0072] According to the polyamic acid varnish composition pertaining tothe present invention, a specific imidazolyl-diaminoazine is containedas an additive, and hence no problems such as blooming or the formationof vapor residue substances occur during the imidation of polyamic acid,nor any electrical migration, and furthermore, it is possible to form apolyimide insulating layer which demonstrates satisfactory adhesivestrength with respect to the copper foil.

[0073] The entire disclosure of the specification and claims of JapanesePatent Application No. 11-360041 filed on Dec. 17, 1999 is herebyincorporated by reference.

What is claimed is:
 1. A polyamic acid varnish composition, comprising apolyamic acid obtained by the addition polymerization of an aromaticdiamine and an aromatic acid dianhydride, and a solvent, wherein saidpolyamic acid varnish composition contains as an additive animidazolyl-diaminoazine as represented by the Formula (1)

(In the formula, A is an imidazolyl group as represented by Formulae(1a), (1b) or (1c).

R¹ is an alkylene group, and m is 0 or
 1. R² is an alkyl group and n is0, 1, or
 2. R³ and R⁴ are alkylene groups, and p and q are respectively0 or
 1. B is an azine, diazine or triazine residue.).
 2. The polyamicacid varnish composition according to claim 1 , wherein B is a triazineresidue.
 3. The polyamic acid varnish composition according to claim 1 ,wherein the imidazolyl-diaminoazine as represented by Formula (1) isselected from the group consisting of:2,4-diamino-6-[2-(2-methyl-1-imidazolyl)ethyl]-s-triazine;2,4-diamino-6-[2-(2-ethyl-1-imidazolyl)ethyl]-s-triazine;2,4-diamino-6-[1-(2-undecyl-1-imidazolyl)ethyl]-s-triazine;2,4-diamino-6-[2-(2-imidazolyl)ethyl]-s-triazine;2,4-diamino-6-[2-(1-imidazolyl)ethyl]-s-triazine;2,4-diamino-6-(2-ethyl-4-imidazolyl)-s-triazine;2,4-diamino-6-[2-(4-methyl-1-imidazolyl)ethyl]-s-triazine;2,4-diamino-6-(2-ethyl-5-methyl-4-imidazolyl)-s-triazine;2,4-diamino-6-(4-ethyl-2-methyl-1-imidazolyl)-s-triazine;2,4-diamino-6-[3-(2-methyl-1-imidazolyl)propyl]-s-triazine;2,4-diamino-6-[4-(2-imidazolyl)butyl]-s-triazine;2,4-diamino-6-[2-(2-methyl-1-imidazolyl)propyl]-s-triazine;2,4-diamino-6-[1-methyl-2-(2-methyl-1-imidazolyl)ethyl]-s-triazine;2,4-diamino-6-[2-(2,5-dimethyl-1-imidazolyl)ethyl]-s-triazine;2,4-diamino-6-[2-(2,4-dimethyl-1-imidazolyl)ethyl]-s-triazine; or2,4-diamino-6-[2-(2-ethyl-4-methyl-1-imidazolyl)ethyl]-s-triazine. 4.The polyamic acid varnish composition according to claim 1 , wherein thepolyamic acid varnish composition contains the imidazolyl-diaminoazinein a ratio of 0.1 to 10 wt parts per 100 wt parts polyamic acid.
 5. Thepolyamic acid varnish composition according to claim 1 , wherein thearomatic diamine is selected from the group consisting of4,4′-diaminodiphenylether, paraphenylenediamine,4,4′-diaminobenzanilide, 4,4′-bis(p-aminophenoxy)diphenylsulfone and2,2-bis[4-(4-aminophenoxy)phenyl]propane.
 6. The polyamic acid varnishcomposition according to claim 1 , wherein the aromatic acid dianhydrideis selected from the group consisting of pyromellitic dianhydride,3,4,3′,4′-biphenyltetracarboxylic dianhydride,3,4,3′,4′-benzophenonetetracarboxylic dianhydride, and3,4,3′,4′-diphenylsulfonetetracarboxylic dianhydride.
 7. A flexibleprinted board having a polyimide insulating layer furnished on a metalfoil, wherein the polyimide insulating layer is formed by forming a filmof the polyamic acid varnish composition according to claim 1 on themetal foil, followed by imidizing.